If the car seats are airline approved (most seats with an internal 5pt harness are, check the labeling on the side of the seat), I would take those seats on board the plane and used them. The FAA strongly recommends using a CRS aboard planes until at least 40lbs. In addition, according to the research I've done, airlines consider car seats to be "fragile" and will not reimburse the owner for them if they are damaged in transit. We've also had bags misrouted more than once, and if one of those bags had been a car seat, then we would have been in a real bind when it came to leaving the airport (in both cases, we did not get the bags back for 2-4 days). The one time I did risk checking a car seat, we only needed it for a couple of days in the middle of our trip (we used DME to and from the resort) so if the car seat had been damaged or lost, one of us could have driven to a store to buy a replacement if we had to). It was sort of a calculated risk if you will.
What I would do in your shoes is buy two car seat bags and put two seats in each bag. Another option would be to use an inexpensive metal luggage cart...stack them on that and bungee them on. OR, if you'll have a stroller, you could stack the car seats on that, and let your kids walk, if they are big enough to walk through the airport. If one of your kids is a baby or toddler, you can use a sling or carrier instead of the stroller. If one or more of the seats are just belt positioning boosters, those will have to be checked and I would definitely suggest gate checking over baggage check. This way, you know they made it on the same plane as you, and they will be loaded last, with the strollers and wheelchairs. Backless boosters can fit in the overhead bin, and sometimes folding HBB's (like the Compass/First Years brand makes) will fit in the overhead bin too, but that will count as a carry on item).
hth